CBM, even if one is vegetarian or eats meat raised without growth hormones or antibiotics, fat cells in the human body produce hormones. I suspect that once the body goes over a certain level/percentage of fat cells (possibly a range), then metabolism starts a loop of dysfunction that's increasingly difficult to change.
I tend to think that being overweight is a symptom of other problems. Metabolism is complex. It appears that we are well adapted to survive famine, but much less well adapted to a post-industrial lifestyle of ample food and minimal exercise. Other complicating factors such as growth hormones in meat, or high-fructose corn syrup in many foods, undoubtedly are unhelpful in bringing one's metabolism back into proper function and general balance.
